A CubeSat with a prototype scintillating detector with a sensitive volume of Gadolinium-Aluminum-Gallium-Garnet crystal is being developed with a possible launch date of 2025. Its purpose is to characterize the background signals that mimic the neutrino interaction that the νSOL (Neutrino Solar Orbiting Laboratory) team is looking for. An important part of the characterization of the backgrounds and the expected real signal is understanding the gamma ray photopeak efficiency of the prototype detector when compared to simulations performed in Geant4. To this end we have used three techniques to do a measurement of the gamma ray efficiency compared to simulation. The first is using electron capture sources that emit an X-ray before prompt emission of a de-excitation gamma ray, specifically 65Zn and 54Mn. The second is using a β+ decay source wherein a positron annihilates on an atomic shell electron producing two back-to-back 511 keV gammas followed promptly by a de-excitation gamma, specifically 22Na. The third is using a gamma cascade of two near-simultaneous de-excitation gammas from the same nucleus, specifically from 60Co decay. © 2024 Elsevier B.V.